FIG. 1 diagrammatically illustrates a conventional push-pull DC-AC converter which incorporates a resonant LC tank circuit in its switched output path, so that its output voltage will vary in a generally sinusoidal manner. To this end, the converter comprises an upper controlled switching device 10, shown as a field effect transistor (FET), which has its source-drain path coupled between a prescribed reference voltage (e.g., ground (GND)) 11, and a first end 13 of an upper coil portion 15 of a center-tapped primary winding 17 of an output transformer 19. In a complementary manner, a lower controlled switching device 20, also shown as a field effect transistor, has its source-drain path coupled between the prescribed reference voltage (GND) 11 and a first end 21 of a lower coil portion 23 of the transformer's primary winding 17. The transformer's primary winding 17 has a center-tap 25 coupled to a voltage source 27. A capacitor 31 is coupled across the two ends 11 and 21 of the primary winding 17 and forms therewith a resonant LC tank circuit, the functionality of which is to cause the switched output of the DC-AC converter to have a generally sinusoidal shape. This sinusoidally varying output is extracted at the output terminals 33 and 35 of a secondary transformer winding 37, which is mutually coupled with the respective upper and lower coil portions 15 and 23 of primary winding 17.
In operation, with the lower FET 20 being OFF, the upper FET 10 is turned ON by the application of a switching waveform to its control (gate) input 12, so that a current flow path is provided from the voltage source 27, through the upper coil portion 15 of the transformer's primary winding 17 and through the source-drain path of the FET 10. This switched ‘push’ current through the upper coil portion of the transformer's primary winding 17, in turn, produces an output voltage waveform across the transformer's output terminals 33 and 35. This output voltage waveform has a first polarity (e.g., positive), in accordance with the polarity of the magnetic coupling between the upper coil of the primary winding and the secondary winding of the transformer.
Thereafter, the upper FET 10 is turned OFF and the lower FET 20 is turned ON by the application of a switching waveform to the control (gate) input 22 of the lower FET, so that a current flow path is provided from the voltage source 27, through the lower coil portion 23 of the transformer's primary winding 17 and through the source-drain path of the FET 20. This switched ‘pull’ current through the lower coil portion of the transformer's primary winding, in turn, produces an output voltage waveform across the output terminals 33 and 35. This output voltage waveform has a second polarity (e.g., negative), in accordance with the polarity of the magnetic coupling between the lower coil of the primary winding and the secondary winding of the transformer.
For optimal performance and efficiency, the frequency of the controlled ON-OFF switching of the two FETs corresponds to the resonant frequency of the LC tank circuit formed by primary winding 17 and the capacitor 31, and the switching of the FETs is controlled so as to occur at points in time when the voltage across the FETs (or at the opposite ends of the primary winding) equals zero. Namely, the two coil portions of the transformer's primary winding are effectively ‘pumped’ by zero voltage-switched FETs at the resonant frequency of the LC tank circuit formed by primary winding 17 and the capacitor 31, so as to produce a very sinusoidal output waveform at output terminals 33 and 35.
Where the parameters of the desired AC output voltage can be supplied by a single push-pull DC-AC converter stage as described above, the circuit of FIG. 1 works substantially well. However, in many applications it is desired to obtain a larger amplitude AC output voltage than is produced by the single stage configuration of FIG. 1. Such a larger voltage is customarily realized by differentially combining the outputs of a pair of stages configured as shown in FIG. 1. As a non-limiting example, it may be desired to employ a pair of the circuits shown in FIG. 1 to drive opposite ends of a cold cathode fluorescent lamp (CCFL), of the type found in large scale display applications, such as large scale television screens, which require an associated set of high AC voltage-driven cold cathode fluorescent lamps (CCFLs) mounted directly behind the screens for backlighting purposes. Indeed, large LCD panels require relatively large numbers (e.g., on the order of ten to forty) of such CCFLs for uniform backlighting.
Unfortunately, the inherent differences in the parameters of each DC-AC converter's components cause the two output voltage waveforms produced thereby to have slightly different frequencies. This leads to two problems. First, due to variations in the components of the two tank circuits, which are not a priori known, the resonant frequencies of the two circuits are not exactly the same, resulting in the output voltage waveform containing beat frequencies, which are undesirable. Secondly, as shown in the encircled portion of the output voltage waveform of FIG. 2, this slight difference in resonant frequencies prevents efficient zero voltage-switching of both of the push-pull switching stages that drive the tank circuits, thereby reducing the operational efficiency of one or both converters.